A stent is commonly used alone or in conjunction with angioplasty to ensure patency through a patient's stenosed vessel. Stents overcome the natural tendency of the vessel walls of some patients to restenose after angioplasty. A stent is typically inserted into a vessel, positioned across a lesion, and then expanded to create or maintain a passageway through the vessel, thereby restoring near-normal blood flow through the vessel.
A variety of stents are known in the art, including self-expandable and expandable stents, as well as wire braid stents. One such stent is described, for example, in U.S. Pat. No. 4,733,665 to Palmaz. Expandable stents are typically delivered to treatment sites on delivery devices, such as balloon catheters or other expandable devices. Balloon catheters may comprise a balloon having a collapsed delivery configuration with wings that are wrapped and folded about the catheter. An expandable stent is then disposed in a collapsed delivery configuration about the balloon by compressing the stent onto the balloon. The stent and balloon assembly may then be delivered, using well-known percutaneous techniques, to a treatment site within the patient's vasculature, for example, within the patient's coronary arteries. Once the stent is positioned across a lesion at the treatment site, it is expanded to a deployed configuration by inflating the balloon. The stent contacts the vessel wall and maintains a path for blood flow through the vessel.
Significant difficulties have been encountered during stent delivery and deployment, including difficulty in maintaining the stent on the balloon and in achieving symmetrical expansion of the stent when deployed. Several techniques have been developed to more securely anchor the stent to the balloon and to ensure more symmetrical expansion. These include plastically deforming the stent so that it is crimped onto the balloon, and sizing the stent such that its internal diameter provides an interference fit with the outside diameter of the balloon catheter. Such techniques have several drawbacks, including less than optimal securement of the stent to the balloon. Consequently, the stent may become prematurely dislodged from the balloon during advancement of the stent delivery system to the treatment site.
Stent delivery systems utilizing a removable sheath disposed over the exterior surface of the stent, which is removed once the stent is positioned at the treatment site, have also been proposed, for example, in U.S. Pat. No. 5,690,644 to Yurek et al. Such systems may be used with or without retainer rings and are intended to protect the stent during delivery and to provide a smooth surface for easier passage through the patient's vasculature. However, the exterior sheath increases the crossing profile of the delivery system while decreasing flexibility, thereby decreasing the ability of the device to track through narrowed and tortuous anatomy.
U.S. Pat. No. 6,106,530 to Harada describes a stent delivery device comprising a balloon catheter having stoppers disposed proximal and distal of a balloon on to which a stent is affixed for delivery. The stoppers are separate from the balloon and maintain the stent's position in relation to the balloon during delivery. As with the removable sheaths discussed previously, the stoppers are expected to increase delivery profile and decrease flexibility of the stent/balloon system.
U.S. Pat. No. 6,110,180 to Foreman et al. provides a catheter with a balloon having pre-formed, outwardly-extending protrusions on the exterior of the balloon. A stent may be crimped onto the balloon such that the protrusions extend into the gaps of the stent, thereby securing the stent about the balloon for delivery. A drawback to this device is the added complexity involved in manufacturing a balloon with pre-formed protrusions. Additionally, if the protrusions are not formed integrally with the balloon, there is a risk that one or more of the protrusions may detach during deployment of the stent. The protrusions may also reduce flexibility in the delivery configuration, thereby reducing ability to track through tortuous anatomy.
U.S. Pat. No. 5,836,965 to Jendersee et al. describes a hot-molding process for encapsulating a stent on a delivery system. Encapsulation entails placement of the stent over a balloon, placement of a sheath over the stent on the balloon, and heating the pressurized balloon to cause it to expand around the stent within the sheath. The assembly is then cooled while under pressure to cause the balloon to adhere to the stent and to set the shape of the expanded balloon, thereby providing substantially uniform contact between the balloon and the stent. This method also provides a substantially uniform delivery profile along the surface of the encapsulated balloon/stent assembly.
A significant drawback of Jendersee's encapsulation method is the need to heat the balloon in order to achieve encapsulation. Such heating while under pressure may lead to localized plastic flows resulting in inhomogeneities along the length of the balloon including, for example, varying wall thickness. Varying wall thickness may, in turn, yield areas of decreased strength that are susceptible to rupture upon inflation of the balloon during deployment of the stent. Additionally, heating and cooling increases the complexity, time, and cost associated with affixing the stent to the balloon.
U.S. Pat. No. 5,976,181 to Whelan et al. provides an alternative technique for stent fixation involving the use of solvents to soften the balloon material. In this method, the stent is disposed over an evacuated and wrapped balloon while in its compact delivery configuration. A rigid tube is then placed over the stent and balloon assembly, and the balloon is pressurized while the balloon is softened by application of a solvent and/or heating. The rigid tube prevents the stent from expanding but allows the balloon to deform so that its surface projects through either or both of the interstices and ends of the stent. Softening under pressure molds the balloon material such that it takes a permanent set into the stent. Once pressure is removed, the stent is interlocked with the surface of the balloon, providing substantially uniform contact between the balloon and the stent and a substantially uniform delivery profile.
As with the technique in the Jendersee patent, the technique in the Whelan patent has several drawbacks. Chemically softening the balloon material under pressure is expected to introduce inhomogeneities along the length of the balloon, such as varying wall thickness, which again may lead to failure of the balloon. Additionally, chemical alteration of the balloon, via application of a solvent to the surface of the balloon, may unpredictably degrade the mechanical characteristics of the balloon, thereby making accurate and controlled deployment of a stent difficult. Softening also adds cost, complexity, and time to the manufacturing process.
In view of the drawbacks associated with previously known methods and apparatus for loading a stent onto a stent delivery system, it would be desirable to provide methods and apparatus that overcome those drawbacks.
It would be desirable to provide methods and apparatus for loading a stent onto a stent delivery system that enhance positional stability of the stent during delivery.
It would further be desirable to provide methods and apparatus for loading a stent onto a stent delivery system wherein the delivery system comprises a crossing profile and flexibility suitable for use in tortuous and narrowed anatomy.
It would still further be desirable to provide methods and apparatus for loading a stent onto a stent delivery system that provide a substantially symmetrical expansion of the stent at deployment.
It would also be desirable to provide methods and apparatus for loading a stent onto a stent delivery system that do not unpredictably modify the mechanical characteristics of the balloon during fixation of the stent to the balloon.